1. Field of the Invention
The present invention relates to a circuit for automatically adjusting the input sensitivity of an audio signal in a monitor. In particular, the present invention relates to a circuit for automatically adjusting the input sensitivity of an audio signal in a monitor capable of preventing the audio signal processing circuit of the monitor from being damaged, by setting a proper reference value for the audio input sensitivity and by applying the input audio signal with a proper level in accordance with the size of the audio input of the personal computer, even if an overinput of an audio signal is inputted thereto due to the output of the sound card or the loudspeaker of the personal computer.
2. Description of the Related Art
The audio signal processing circuit of the monitor is generally classified into a circuit having a two-system input terminals consisting of an input from a loudspeaker and a line input terminal, and a circuit having a line input terminal only.
Referring to FIG. 1, the conventional audio signal processing circuit of the monitor having the loudspeaker input terminal and the line input terminal, comprises an input sensitivity adjusting section 1 for adjusting the input sensitivity of the audio signal applied to the loudspeaker input terminal and the line input terminal to which the audio signal via the sound card of the personal computer is inputted, a pre-amplifier 2 for pre-amplifying the audio signal controlled by the input sensitivity adjusting section 1, a power amplifier 3 for amplifying the audio signal outputted from the pre-amplifier 2, and a loudspeaker 4 for outputting the audio signal amplified through the pre-amplifier 3. Reference numerals R1 and R2 indicates resistors, respectively, and VR1 indicates a variable resistor for adjusting volume.
According to the conventional audio signal processing circuit of the monitor as described above, when the audio signal is applied to the line input terminal or the loudspeaker input terminal through the sound card of the personal computer, the applied audio signal is inputted to the input sensitivity adjusting section 1, as shown in FIG. 1. The audio signal inputted into the input sensitivity adjusting section 1 is attenuated by a predetermined level by the resistors R1 and R2, and then inputted to the pre-amplifier 2. The audio signal inputted to the pre-amplifier 2 is pre-amplified and inputted to the power amplifier 3. The amplified audio signal is inputted to power-amplified and by the power amplifier 3, and outputted to the loudspeaker 4, so that the loudspeaker 4 radiates outwardly the audio signal inputted.
Since the audio signal processing circuit of the monitor is so constructed that the attenuation rate between the loudspeaker input and the line input is in the range of about 5:1 to 10:1, a user can select and connect the input terminal mating with the output condition of the sound card of the personal computer.
Meanwhile, the conventional audio signal precessing circuit of the monitor having the line input system comprises, as shown in FIG. 2, an input sensitivity adjusting section 5 for adjusting the input sensitivity of the audio signal applied to the line input terminal, to which the audio signal via the sound card of the personal computer is applied, a pre-amplifier 6 for pre-amplifying the audio signal adjusted by the input sensitivity adjusting section 5, a power amplifier 7 for amplifying the audio signal outputted from the pre-amplifier 6, and a loudspeaker 8 for outputting the audio signal amplified by the power amplifier 7. Reference numerals R3 and R4 indicate resistors, respectively, and VR2 indicates a variable resistor for adjusting volume.
According to the conventional audio signal processing circuit of the monitor as described above, when the audio signal is applied to the line input terminal from the sound card of the personal computer, the applied audio signal is inputted into the input sensitivity adjusting section 5, as shown in FIG. 2. The audio signal inputted to the input sensitivity adjusting section 5 is attenuated by the predetermined level by the resistors R3 and R4, and then inputted to the pre-amplifier 6. The audio signal inputted to the pre-amplifier 6 is pre-amplified to be inputted to the power amplifier 7. The amplified audio output signal is power-amplified and the power-amplified audio signal is inputted to the loudspeaker 8. The loudspeaker 8 radiates outwardly the audio signal.
Since the audio signal inputted to the input sensitivity adjusting section 5 is adjusted within the range of 0.5 to 0.7 Vrms by the attenuating function of the resistors R3 and R4, the user has to connect line output terminal to maintain a proper input sensitivity.
Because the conventional audio signal processing circuit of the monitor having only one-system line input terminal is not compatible with the sound card of the personal computer having only a speaker output terminal, which is widely used, the output voltage from the speaker terminal becomes about 2 Vrms that is 3 or 4 times higher than the rated input voltage. Thus, the pre-amplifier is overloaded and the distortion is increased, causing the loudspeaker or headphone to be damaged. Also, even in the case of having the two-system input terminal, i.e., the speaker input terminal and the line input terminal, since the input terminal should be properly selected to mate with the output condition of the sound card of the personal computer, it is very inconvenient in use.